pipe.go

// Copyright (c) 2018, Sylabs Inc. All rights reserved.
// This software is licensed under a 3-clause BSD license. Please consult the
// LICENSE.md file distributed with the sources of this project regarding your
// rights to use or distribute this software.

package exec

import (
	"fmt"
	"os"
	"os/exec"
	"syscall"
	"unsafe"
)

// Pipe execute a command with arguments and pass data over pipe
func Pipe(command string, args []string, env []string, data []byte) error {
	pipeEnv, err := SetPipe(data)
	if err != nil {
		return err
	}

	env = append(env, pipeEnv)
	err = syscall.Exec(command, args, env)
	if err != nil {
		return fmt.Errorf("command %s execution failed: %s", command, err)
	}

	return nil
}

// PipeCommand creates an exec.Command struct which will execute the starter binary
func PipeCommand(command string, args []string, env []string, data []byte) (*exec.Cmd, error) {
	pipeEnv, err := SetPipe(data)
	if err != nil {
		return nil, err
	}

	env = append(env, pipeEnv)

	c := &exec.Cmd{
		Path: command,
		Args: args,
		Env:  env,
	}
	return c, nil
}

// setPipe sets a pipe communication channel for JSON configuration data and returns
// the file pointer to the read pipe
func setPipe(data []byte) (*os.File, error) {
	r, w, err := os.Pipe()

	if err != nil {
		return nil, fmt.Errorf("failed to create pipe: %s", err)
	}

	rfd := r.Fd()
	wfd := w.Fd()

	pipeFd, err := syscall.Dup(*(*int)(unsafe.Pointer(&rfd)))
	if err != nil {
		return nil, fmt.Errorf("failed to duplicate pipe file descriptor: %s", err)
	}

	if n, err := syscall.Write(*(*int)(unsafe.Pointer(&wfd)), data); err != nil || n != len(data) {
		return nil, fmt.Errorf("failed to write data to stdin: %s", err)
	}

	if _, _, err := syscall.Syscall(syscall.SYS_FCNTL, rfd, syscall.F_SETFD, syscall.FD_CLOEXEC); err != 0 {
		return nil, fmt.Errorf("failed to set close-on-exec on read pipe: %s", err.Error())
	}

	if _, _, err := syscall.Syscall(syscall.SYS_FCNTL, wfd, syscall.F_SETFD, syscall.FD_CLOEXEC); err != 0 {
		return nil, fmt.Errorf("failed to set close-on-exec on write pipe: %s", err.Error())
	}

	return os.NewFile(uintptr(pipeFd), "pipefd"), err
}

// SetPipe sets the PIPE_EXEC_FD environment variable containing the JSON configuration data
func SetPipe(data []byte) (string, error) {
	pipe, err := setPipe(data)
	if err != nil {
		return "", err
	}
	return fmt.Sprintf("PIPE_EXEC_FD=%d", pipe.Fd()), nil
}